Music laboratory

ABSTRACT

A system of class intercommunication which provides: 1. INDIVIDUAL CHANNELS BETWEEN A TEACHER AND A. ANY STUDENT OF THE CLASS, OR B. ANY OF SEVERAL PRESELECTED GROUPS OF STUDENTS OF THE CLASS, OR C. THE ENTIRE CLASS. 2. PROVISION FOR ENSEMBLE PLAYING WITHIN ONE OR MORE OF THE GROUPS. 3. PROVISION FOR GROUP INSTRUCTION FROM AUDIO AIDS, ALL UNDER CONTROL OF THE TEACHER ONLY. Programming is achieved by rotary group programming switches, which determine the character of the facilities, e.g., individual, audio aids, and ensemble, by interlocked group selection switches which enable selection of one group to the exclusion of the others, by further interlocked selection switches which enable selection of an individual student to be monitored or privately instructed, and by a switch which enables the teacher to provide any or all groups with instruction simultaneously. A minimum number of switch actuators is provided, physically grouped for easy access and control within the span of one hand.

United States Patent Inventors Daniel W. Martin; Robert G. Morgan, both 01 Cincinnati,

Ohio [21] Appl. No. 719,126 [22] Filed Apr. 5,1968 I [45] Patented July 27, 1971 [73] Assignee 0.11. Baldwin Company Clndnnatl, Ohio [54] MUSIC LABORATORY v 42 Claims, 3 Drawing Figs.

[52] U.S.Cl. .L 84/470, 35/5 [51] lnt.Cl ..G09b 15/08 [50] Field 01 84/470, 477, 478, 1.01; 35/5, 6, 35.3

[56] 1 References Cited UNITED STATES PATENTS 3,00l,431 9/1961 Andersen 84/470 X 3,184,549 5/1965 Auemheimer 35/35 X 3,470,785 10/1969 Shallenberger etal. 84/470 3,475,833 l [/1969 Andersen 84/478 X 3,478,159 11/1969 Olson 84/470 X Primary Examiner-Richard B. Wilkinson Assistant Examiner-John F. Gonzales AtromeysW. H. Breunig and Hurvitz, Rose and Greene ABSTRACT: A system of class intercommunication which provides: 7

l. individual channels between a teacher and a. any student of the class. or by; any of several preselected groups of students ofthe class. or c. the entire class. 2. provision for ensemble playing within one or more of the groups.

3. provision for group instruction from audio aids, all under control of the teacher only. M V V Programming is achieved by rotary group programming switches, which detennine the character of the facilities, e.g., individual, audio aids, and ensemble, by interlocked group selection switches which enable selection of one group to the exclusion of the others, by further interlocked selection switches which enable selection of an individual student to be monitored or privately instructed, and by a switch which enables the teacher to provide any or all groups with instruction simultaneously. A minimum number of switch actuators is provided, physically grouped for easy access and control within the span of one hand.

SHEET 2 OF 3 PATENTED JUL27 I97! v Algal V 4. mvsmoras DANNEL U). MQRTlNr- ROBERT G. MORGAN ATTORNEYS MUSIC LABORATORY BACKGROUND OF THE INVENTION Language laboratories have been extensively employed for many years. More recently, some of the techniques heretofore applied to language training have been extended to the teaching of music. To this end musical instruments are employed, which produce electrical signals but are, or can be, silent acoustically. It is desirable for one instructor to instruct a large number of pupils, say 24, for example. For convenience these may be divided into four groups, identified as A, B, C, D, respectively, each of which includes six students. It is then required that the instructor be enabled tolisten to any one student, that the student be enabled to hear the playing of the instructor, or music deriving from one or more external sources, and that two-way voice communication between the student and the instructor be available at the option of the teacher. Furtherthe students may play in ensemble, by group, or groups and also an intercommunication facility must exist in the sense that the students within a group must hear each other, and the teacher may hear the students of any group. In one system known to us, this has led to provision of [6 pushbuttons and five three-position switches, on a control panel available to the instructor, and to a two-position switch at each student position, for solution of the communication problem. These switches must be manipulated in various combinations, to achieve the various of the desired communication objectives and flexibility of end result is achieved by complexity of manipulation, which may be unacceptable to the many music instructors who are electronically unsophisticated. The present invention concerns itself with the provision of a simpler control system, arranged physically to facilitate manipulation by virtue of compactness of a switch actuator array, and psychologically to facilitate understanding of the available functions of the switches and how these may be achieved. The present system also relates to simplification of electrical switching circuitry, in achieving a wide variety of possible interconnections It is a feature of the system that all intercommunication functions are under control of the teacher, in the sense that no student is called upon to operate controls to effect group relationships. Moreover no student can elect to call the teacher while the teacher is engaged with another student. Another feature is that the total number of switching elements is reduced in the present system in comparison with the number required in other systems of class music instruction known to us, which simplifies wiring and operation and reduces maintenance problems.

SUMMARY OF THE INVENTION An intercommunication between a teacher and (a) individual students of a class, (b) a group within the class, or (c) the entire class, coupled with provision for distributing audio signals from magnetic tape or record reproducers to groups of students, or to the class, and for ensemble playing by one or more groups. These functions are selectively accomplished, in the case of a four group class, by four four-position rotary switches and an array of 12 pushbuttons. One rotary switch applies to each group, and its four positions determine whether the group is programmed for individual instruction, for reception of recorded music derived from either of two recorders, or for ensemble playing. The pushbuttons enable selection of an individual of any group for private communication, selection of a group for group instruction with optional simultaneous monitoring of any individual within the group, or selection of one to four groups for simultaneous instruction.

BRIEF DESCRIPTION OF THE DRAWINGS The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific system in FIG. 1, showing the physical arrangement of controls.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present system is designed in the preferred embodiment, to accommodate a class of 24 students, divided into four groups of six students per group. Each student is provided with an outgoing or a line, as 10, and an incoming ofB line 11. These proceed via isolating resistances 12, 13 to student headphones 14. The a line is connected via switch contacts 15 to a student amplifier l6 and electropiano pickups 17, or in the alternative, via switch 18, to the student's microphone 20. Accordingly, the student can provide either oral or piano signal, at will, to the outgoing or a line 10. The student can hear his own voice or piano in headphones 14, or may transfer these to a loudspeaker 21 via selective switch 15. Resistors 12, I3 isolate the a line from the [3 line, so that signals incoming on the 3 line 11 do not proceed out via the a line 10. The headphones 14 are connected between the junction of resistors l2, 13, so that the student may hear himself and/or incoming signals.

The teacher is provided a teacher station which is provided with headphones 30 and with a microphone 31 and electropiano 32 as well as a tape recorder 100, amplifiers 36 and 33, and various controls, the positions and functions are illustrated in Fig. 2 of the drawing. The microphone 31 is connected through a preamplifier 33 to a teachers power amplifier 36, to which the output of piano pickups 32 is connected at will. The output of preamplifier 33 is unaffected when switch 35 is connected to contacts 34, but is grounded by contact to 34a when the teacher is listening only, to minimize noise which might disturb the student. The output of teachers amplifier 36 is connected to a line 40, which proceeds on to the several students, via switch 41, and which connects by alternate routes eventually to the teachers headphone circuit. The teachers headphone 30 is connected between the midpoint of isolating resistors R R and ground, and is thereby connected to the teacher's amplifier 36, or to the group amplifier G and to a selected students a line.

The teacher can communicate with the entire class of 26 students, by depressing the class switch 43. Thereby a circuit is completed to contacts 44 and via resistances 45, to lines 46, 47, 48 and 49. These lines are associated, respectively, with switches relating respectively to groups of students, each group containing six students, and the groups being identified respectively as groups A, B, C, D. Only the switches of group A are illustrated. The arrangement is duplicated for the several groups, and is illustrated only once, to simplify the exposition. While the class switch 43 is depressed the teacher hears the output of teacher amplifier 36 via lines 4%, 40b, and 40c.

Line 46 proceeds to amplifier input terminal 48, of amplifier G, specific to group A. Each group is provided with a separate group amplifier. We now desire that the teachers signals proceed to all the students of the class. The rotary switch decks R,, R R ,...for each group are controlled by a single knob. If this knob is turned to its a, b or d position, the output of amplifier G then proceeds via line 50, line 51 and the a, b or d contact of switch deck S to terminal line 1 B. Deck R, is connected to line 2 B, deck R, to line 36, and so on for the decks R R and R,, connecting student headphone circuits to the group amplifier output. Deck R7 is not connected to line 50, but is connected to audio aid terminals 52, 53, assumed not operative at this time. Similar reasoning applies to the switch decks of groups B, C and D, so that any selected group of students can now hear the teacher, each via Two additional audio input lines are provided at 52 and 53, which are connected to the input of amplifier G, via R and line 48. Lines 52 and 53 may be connected, respectively, to tape recorders or phonographs so that all the students may hear these, as described hereinafter. Also the d, or ensemble position of switchesS, and R, to R are all connected to lines 50 and 51, so that any selected group or groups of students can hear the teacher in this position.

There are therefore three positions (a, b, and d of the RS switches for which the teacher can transmit to the entire class. In positions a, b or c a selected student can transmit to the teacher. In two of these positions, a and b, audio aids can be provided by the teacher. However, the RS switchesillustrated are all controlled by one knob, and each group has a knob. it follows that, for example, group A can listen to one audio aid, group B to another audio aid and groups C and D to the teacher alone by setting the rotary switches for these groups. Switch R7 serves to select input for group amplifier G from one of the audio aids, in its a or b positions, and in its d position it selects the ensemble output of all the students of a group from line 60, which is connected through relatively high resistances 80 to all the or lines of the group.

if it is desired that the teacher transmit only to a selected group, the rotary switches R, S are again set to their a or b positions. instead of closing class switch 43, one of group switches A, B, C, D, is closed. Assume that switch A is closed. The signal now proceeds via contacts 60 only from switch 41. Signal proceeds to line 46, as before, and to the input of amplitier G, but not to lines 47, 48, 49, as in the case of class instruction. From line 46 signal proceeds only to the group amplifier G supplying signal 1a-six lines of group A. Again this group can h ar signal input to terminals 52 and 53, deriving from tape or disc reproductions devices, according to the setting of R Audio terminals 52, 53 proceed to the a and b contacts of switch R-,, respectively, so that these may be selectively applied to the group, depending on whether the RS switches are in a or b positions, but the teacher can be connected to the group in either case when switch 41 is in instruct" or call position.

For any group function of Group A, fo example, the teacher hears her own voice and piano (only if switch 41 is operated in instruct or call") via line 40, contacts 60, line 46, group amplifier G, line 65, contact 66, and line 400. By thus routing the teacher signals through the group amplifier to her headset, one insures that she hears herself over only one path while instructing a group.

Facility'is provided for ensemble playing for all the instruments of a group. For this purpose, the RS switches are set to the d position. if the A group is selected for this purpose, the switching system must interconnect all the or lines with all the B lines, and to the teacher when desired. The la line now proceeds to switch S,d and thence via line 60, which leads to contact RA. The rotor of R, connects to line 46, which leads to the input of amplifier G. The output of the amplifier G proceeds via line 51 and switch 8 d to line 16 The output of amplifier G also proceeds to line 50, and the rotors of R R R,, R R lead respectively to lines 25 to 63, respectively, while R,d leads to line 65, contacts 66, line 40c and the teacher headphones 30. The teacher can then hear all the students of group A and they can hear each other.

The teacher can talk to the entire group, in the path 31 (her microphone), 33, 35, 36, amplifier line 40, switch 41, contact 60, line 46, to amplifier input 48. The paths following the amplifier G have hereinabove been traced, and the description is not repeated.

For purpose of individual instruction the SR switches are set to position c. Assume theteacher desired to communicate with student A1, i.e., student (ll of group A. Switch 70 and switch A- would be actuated. Line la is now connected ,via contact S,c to line 68a, contacts 61 and 70 and line 67 to the teacher's headphones. Note that this path contains no amplifier. The teacher's signal proceeds from amplifier 36 to switch 41, and thence to line 75, contacts 76, contacts 77, switch 8 c,

to line 13.

The system provides only one set of group selection and individual selection switches, so that one individual of any one group can be selected. Communication with the teacher then proceeds via the A, B, C and D group switches, and the individual switches 70. In such case the remaining groups not selected can play in ensemble or listen to audio aids, i.e., they can operate independently of the teacher.

The system of the invention requires a minimum of controls, assembled in a compact group, i.e., six "individual student" pushbuttons, 90, a cancel" button 91 for these; four group buttons 92 and a class" button 93, canceling group buttons 92, and substituting class button 93, and substituting class" function, located immediately above the individual buttons on panel 94, and four four-position group rotary switches 95 located above the corresponding group buttons. The rotary switches have an ensemble position, d, an individual" position c, and two positions, a and b, dedicated to auxiliary audio input lines. The group buttons 92 select one group at a time, and the class button 93 selects all groups, actuation of a succeeding button resetting the previously set button. Only one group amplifier per group is required. Major controls are located within one rectangle, containing two rows of pushbuttons and four rotary four-position switches, located symmetrically with respect to the pushbuttons. The only other controls involved are a speech level control 96 and a three-position switch 97, which can be set for Listen Only, Call or instruct, at will. The switch 97 corresponds with switches 35, 41 of FIG.

The electropianos of the system all include powerful amplifiers, say 15 watts, so that the electropianos can play through loudspeakers, as 21, FIG. 1A, in which case they simulate the tones of acoustic pianos. The a lines therefore operate at a high audio potential, and this permits signal isolation to be accomplished by passive resistive circuits, and makes available enough signal so that each student can, in the class ensemble situation, transmit a considerable signal to all the 24 sets of headphones, and the teacher. The group amplifiers G act essentially as linear mixers, and are low power amplifiers, perhaps 5 watt. For example, in an ensemble condition, the la line via S,d to a 22K!) resistance and thence via R-,d to the input of amplifier G, and then back via 8 d to line 13. 2200. resistors 12 and 13 in combination with 4 ohm headphones l4 grounded on one terminal, isolate lines 15 and The loudspeakers of the system have impedances of the order of 4-10 ohms, and the amplifiers driving these loudspeakers are capable of supplying to them several watts of power. In the headset mode of operation the output of the electropiano is fed to dummy load 19. The headsets in the system have impedances of only 4 ohms. For equivalent loudness to the listener the power required for listeners headset operating into a listeners cars is a very small fraction of the power supplied to the dummyload. The audio distribution lines of the system then supply power from a low impedance source to remoteswitching, mixing or heatset isolation resistors of considerably higher impedance. The headset isolation resistances, usually 2209 are in series with the low headphone impedances, and a number of these may be connected in parallel across a dummy load. The power lost in the high resistances is not needed by the headphones, and hence power utilization efficiency is sacrificed for desirable operating characteristics. One of these is that as the number of headset loads on a given source varies, the potential across each headset load remains essentially the same.

It is possible, also, to mix signals to a headset from two or more audio power distribution circuits, e.g. an a signal deriving from a student piano and a B signal deriving from a teacher piano output, or a group amplifier. The lossy characteristic of the circuitry including the headphones, in addition to providing the advantages above recited, also provide isolation between lines and prevent'crosstalk, eliminating any need for expensive electronic isolation circuits.

Another advantage of the system is that monitoring is unobtrusive, because switching occurs at high signal level but listening occurs at low signal level. For example, when the teacher switches in to monitor a given student, that student is unaware of the fact because he cannot hear a click when the appropriate switch is closed.

The ensemble mixing circuit is designed to assure that each 5 student hears other individuals in the group at a somewhat lower level than he hears himself from his own amplifier circuit, much as he would in conventional piano groups because of relative proximity to the pianos. However the teacher hears all of the students at the same advantage, just as she would when normally distant from each individual conventional piano. This enables the student to hear himself better, and the teacher to judge ensemble balance.

The many ZZKQ resistances utilized in the system are not I 5 individually discussed nor provided with individual numerals of reference. These are the isolating resistances in signal mixing circuits at the inputs of group amplifiers. in each case, they supply a low current from a high impedance to a relatively lower impedance across the group amplifier input, and also 0 serve to isolate incoming signal lines from each other. For example in the ensemble mixing circuit of FIG. 1A the signal from line 1a comes through S at position d into 22K resistor 80 which is loaded by five parallel circuits to ground, each containing 22K and a dummy load of 4 ohms, effecting a 5 to l voltage division. The same division occurs for signals from 2a,...6a. The combined signals are amplified by group amplifieria i msi q h .8 2s 9f the ra e-t.-. a

in ensem playing, the teacher ha sqheoption of becoming part of the ensemble, or not, as she chooses, and the students can speak to the teacher when she has selected the appropriate group button. Under this condition she cannot call any individual student but can speak to the class.

When the class switches 43 are actuated, any group of the class can be deleted by operating its RS switch to the c position. This provides the ability to include any one, two or three groups in class instruction, the remaining students being in individual study. One or more groups of students can listen to a recording as they play, if the switches RS pertaining to their groups are set to the a or b positions. The teacher may now listen to individual student response to the recording by selecting the appropriate student button, or may listen to none by depressing the cancel button.

The four rotary four-position switches RS, in summary, each enables selection of any one of four facilities, programmed selectively to the four groups. The switches RS are therefore called program switches." The four facilities which are selected according to rotary positions of the switches are:

a. Private practice, which can be monitored unobtrusively by the teacher, or private two-way communication and instruction at the discretion of the teacher. This facility permits the teacher to listen to any individual student, to the exclusion of alLothers, or to talk with that student, or to enable that student to listen to the playing of a passage by the teacher, according as ganged switches 35, 41 are in Listen Only position, or instruct or Call positions. in the Instruct and Listen Only positions the switches 35, 41 lock in, but for the Call position the switch returns to Listen Only position when released.

The a and b positions of rotary switches RS serve to provide the students with music reproduced from either of two magnetic tapes (or disc records) called an "audio signal aid." in the d position of the switches RS the students in a group may hear each other, the ensemble mode. They may also hear the teacher. if all four of switches RS are in the d or ensemble position, each of the four groups can be instructed when the teacher depresses the appropriate group button 92, and all groups can be instructed when the class button 93 is actuated.

Group selection or class selection is accomplished by push buttons A, B, C, D, and 43, all of which are mutually exclu- 7O sive. When an audio aid is selected for the group, actuation of the cancel switch maintains the audio aid for all the members of the group, but disconnects the teacher headset circuit from individual members of the group, which otherwise would "be selected by selective actuation of switches 70, numbered l-6, inclusive, these being mutually exclusive in the sense that closure of any one switch assures that all others will be open. When the teacher performs group selection the audio aid signals may be monitored, the ensemble signal may be monitored, or individual students may be monitored, all within a selected group.

The class selection mode, obtained by closing switch 43, enables the teacher to provide any or all groups with instruction simultaneously, but any group whose programming switch RS is set to individual" will be excluded.

The teacher can thus set each of the four groups independently into any one of the four functions (three group and one individual), and can then speak or play to any one selected group or none, simultaneously hearing what the selected group hears and, if desired, hear also a selected individual within the group selected.

In FIG. 1B is illustrated a tape recording facility which is connected to the center point of two 4.7K resistances 101, across the resistances R1, R2 taken in series. The latter are 2200. resistances. The connection of the tape recorder therefore does not appreciably affect the loudness of sound provided by the earphones 30, but anything heard in the earphones can also be recorded. The connecting lines 102 may also be utilized for connection to observers headphone circuits of the same type as for the teacher.

it will be appreciated that simplifications may be achieved in a system by having only six students, instead of four groups of six, and that this may be accomplished within the principles of the invention as defined in the appended claims.

We claim:

1. A system of group instruction in music, comprising a plurality of student stations having musical instruments providing electrical signals representing music, said plurality of stations being comprised of n groups of stations, each group having m stations,

where m and n are integers,

a headphone for each of said stations,

a signal input line for each of said stations,

a signal output line for each of said stations,

a pair of isolating resistances connected in series between the input line and output line of each of said stations, and

a passive circuit connecting each of said headphones to the junction of the resistances of a different one of said pairs of isolating resistances,

a separate power amplifier associated with each of said instruments for amplifying said electrical signals, each of said power amplifiers having an output, and

means connecting each of said outputs directly to one of said signal output lines.

2. A system of group instruction, comprising a teacher station,

a plurality of student stations divided into groups, each student station being provided with an electrical musical instrument,

a set of group pushbutton switches each operable to connect a group of student instruments operatively to said teacher station to the exclusion of all other groups of a plurality of groups,

a further class pushbutton switch which when actuated overrides all said group pushbutton switches and connects the instruments of all said groups simultaneously operatively to said teacher station, and

a further set of pushbutton switches operative only individually to connect to said teacher station only a single individual of any group selected by one of said group pushbutton switches.

3. A system of class instruction, comprising a teacher station,

a student station including a student electrical musical instrument having tone signal sources, an incoming student signal line proceeding to said student station,

an outgoing student signal line connected to said electrical musical instrument to convey said tone signals from said student station,

a relatively high resistance network joining said lines,

a relatively low resistance headphone connected to ground from a point of said resistance network joined by relatively high resistances of said resistive network to both said lines, and

a source of signal emanating from a teacher station connected directly to said outgoing signal line,

wherein said resistive network and said headphones are arranged and adapted to isolate said lines from each other.

4. The combination according to claim 3, wherein said teacher station includes an electrical musical f-instrument providing a teacher musical signal source,

teacher headphone at'said teacher station,

switching means at said teacher station for at will connecting said outgoing student signal line to Said teacher headphone and said teacher musical signal source to said incoming student signal line. 1

5. A system of intercommunication between (i) a group of student stations, of a class, each station provided a student electrical musical instrument or (2) among individual members of said class or (3) among the entire class'and (4) a teacher station provided with an electrical musical instrument, each student station having a separate incoming and outgoing line, comprising a group amplifier, a teacher station inc ming line, an n position rotary contact switch means having m pairs of decks, wherein n is a number of stationary switch contacts and m is one more than the number of stations of one group, the groups all having equal numbers of stations, and wherein one of the rotary contacts of said m pairs of decks is connected directly to the input of said group amplifier and another to said teacher station incoming line, and the remainder of said rotary contact pairs connected individually with said outgoing and incoming lines of said student stations. wherein each student station outgoing line is connected to a student electrical musical instrument to convey signals therefrom.

6. The combination according to claim 5, wherein is provided an isolative resistive mixer network,

means connected to corresponding ones of said stationary contacts for at will connecting all the incoming lines of a group of said student stations to said isolative resistive mixer network.

7. The combination according to claim 6, wherein is provided means connected to corresponding ones of said stationary switch contacts for at will connecting any selected individual incoming line to said teacher station to the exclusion of all other incoming lines.

8. A system of group instruction in music, comprising a plurality of student stations each including an electrical musical instrument providing student electrical tone signals when played by a student,

a teacher station including a further electrical musical instrument providing teacher electrical tone signals when played by a teacher,

each of said student stations and said teacher station having a headphone responsive to the electrical tone signals provided by the associated electrical musical instrument,

means solely under control of said teacher for routing electrical signals provided by any selected one of saidstudent electrical musical instruments to the headphones of said teacher station and for routing all the electrical signals provided by all said electrical musical instruntents to all said headphones, each of said electrical musical i str ments including an audio amplifier for transmitting said electrical signals, said amplifiers being each of sufficient power output capability to drive a loudspeaker,

a separate loudspeaker individually associated with e agh of means at each of said student stations operative by a student at the station for at will selectively connecting each said loudspeaker or headphones to the output of its individually associated audio amplifier.

9. in a music teaching laboratory having a plurality of electrical musical instruments providing audio tone signals,

an incoming line for each of said instruments,

an outgoing line for each of said instruments,

a relatively high resistance network connecting each incoming line to each outgoing line and providing isolation therebetween for said audio tone signals,

headphones permanently connected between an intermediate point of said resistance network and ground,

an amplifier having an input and an output circuit,

a loudspeaker, and

means for connecting said output circuit selectively directly to said outgoing line or directly to said loudspeaker, said headphones having each relatively low resistance of the order of less than 5 percent of the ohmage of one of said relatively high resistance networks, whereby said headphones provide shunt impedances which enhance the isolation provided by said relatively high resistance networks.

10. In a music laboratory, an incoming teacher line,

a teacher station including a teacher musical instrument providing a source of electrical output signals,

an output teacher line connected to said source,

relatively high isolating resistance connected between said incoming and output lines,

a relatively low input impedance tape recorder for receiving signals from said incoming line, and

a connection from said tape recorder to an intermediate point of said relatively high resistances.

ll. The combination according to claim 10, wherein is additionally provided further relatively high isolating resistance connected between said lines in parallel with said first relatively high resistance, and a second reproducing means connected between a midpoint of said further relatively high resistance and ground, said second reproducing means having relatively low resistance to further decouple said lines while providing a preselected signal level to said second reproducing means.

12. The combination according to claim 10, wherein is pro vided a plurality of pupil stations each including a signal source and a signal receiver connected respectively to an outgoing student line and an incoming student line, and means for selectively and at will connecting said incoming student lines to said outgoing teacher line and said outgoing student lines to said incoming teacher line.

13. The combination according to claim l2, wherein said means for selectively and at will connecting is located at said teacher station.

14. in a music laboratory,

an incoming signal line for first signals,

an outgoing signal line for second signals,

a relatively high resistance network joining said lines and providing isolation therebetween for both said signals,

a relatively low resistance headset connected between an approximate midpoint of said resistance network and ground, said relatively low resistance being selected to provide decoupling between said lines when taken in conjunction with said relatively high resistance network,

a dummy load connected between said outgoing signal line and ground, said dummy load having an impedance of the order of magnitude of said relatively low resistance,

a power amplifier having an input terminal and an output terminal, a loudspeaker, a microphone,

a musical instrument pickup,

means selectively and at will connecting said microphone or said musical instrument pickup to said input terminal, and

means selectively and at will connecting said dummy load or said loudspeaker to said output terminal.

15. The combination according to claim 14, wherein is pro- 17. In a music laboratory,

a plurality of n student stations, where n is an integer, each of said stations including an electrical musical instrument, an outgoing line connected to convey electrical signals from said electrical musical instrument, earphones, an incoming line connected to convey electrical signals to said earphones,

means for conveying electrical tone signals from an electrical musical instrument at each station to the earphones at said each station at a first level, and

means for additionally conveying electrical signals from all of said outgoing lines to the incoming line at each station, whereby each student can hear his own playing distinguishably from the playing of the remaining students.

18. in a music class training system,

a microphone,

a source of music signals deriving from an electrical musical instrument,

an amplifier having an input circuit and an output circuit,

first switch means for selectively connecting said microphone and said source of music signals to said input circuit,

second switch means connected to said output circuit,

an outgoing signal line,

a loudspeaker,

means connecting said second switch means selectively to said outgoing signal line and said loudspeaker,

an incoming signal line,

a pair of relatively high isolating resistances connected between said outgoing and incoming signal lines and a headset connected between the junction of said resistances and a point of reference potential, said headset being of relatively low resistance.

U. In a music laboratory,

a plurality of student stations each including an electrical musical instrument,

a separate outgoing line connected to convey signals from each of said instruments,

a headphone at each of said student stations,

a separate incoming line at each of said student stations connected to convey signal to said headphone at that station,

a passive impedance network means for concurrently conveying signals from an incoming line at each station and from the outgoing line at that station to said headphones while isolating said lines from each other, and

means for at will connecting all said outgoing lines concurrently to all said headphones via said incoming lines.

20. The combination according to claim 19 wherein is provided a teacher's station, said last means being located at said teachers station.

21. The combination according to claim 20, wherein said teacher station includes a source of music signals, and means connecting said source of music signals selectively at will to any one of said incoming lines or to all said incoming lines.

The combination according to claim 20, wherein said teacher station includes a headphone and means at said teacher station for selectively connecting said headphone to any selected one of said outgoing lines and to all said outgoing lines; whereby said teacher may monitor any one of said students or the ensemble of all said students.

23. The combination according to claim 21, wherein said plurality of student stations includes plural groups of stations, and wherein said teacher station includes means connecting said source of music signals to any group of said stations to the exclusion of the other groups.

24. The combination according to claim 22, wherein said plurality of student stations includes plural groups of stations and wherein said teacher station includes means for connecting said source of music signals and said headphone located at said teacher station selectively to any one of said groups of stations.

25. in a music teaching laboratory, an audio power amplifier having an input and an output, said output having a low source impedance and providing plural watts of power,

a loudspeaker,

a source of music signals deriving from a musical instrument, switch means connecting said loudspeaker at will to said output,

headphones having impedance of the order of 4 ohms,

a resistive network connec'table by said switch means from said output to said headphones, said resistive network having impedance of about 220 ohms,

an outgoing audio line, a resistance of the order of the resistance of said loudspeaker connected between said outgoing line and ground,

an incoming line,

a second resistive network of about 220 ohms connected between said incoming line and said headphones.

26. In a music teaching laboratory,

a plurality of student stations arranged in a group,

each of said stations including a source of electrical tone signals deriving from a musical instrument,

a separate outgoing line to convey signals from each of said sources,

a headphone a separate incoming line to convey signals to each of said stations,

a passive network for conveying signals from an incoming line at each station and from the outgoing line at that station to said headphones while isolating said lines from one another; and

means for at will connecting all said outgoing lines from said stations of said group to all incoming lines in said group.

27. A system of group instruction in music, comprising a group of student stations each including an electrical musical instrument providing first electrical signals when said instrument is played by a student,

a teacher station including a source of further electrical signals representative of music to be imitated by said students on said electrical musical instruments,

each of said stations having means for reproducing sound represented by electrical signals,

first switch means (41) solely under the control of said teacher for routing the further electrical signals produced at said teacher station to said means for reproducing at all of said student stations, and second means (61, 62) solely under the control of said teacher for concurrently routing said first electrical signals to said means for reproducing at said teacher's station selectively either concurrently from all said student stations of said group or singly from any one of said student stations of said group, wherein is provided further means (S1--S7, Rl-R7) for selectively at will applying electrical signals from each of said student stations to said means for reproducing of all said student stations.

28. The combination according to claim 27, wherein the source of further electrical signals includes an electrical musical instrument, the combination further comprising a microphone, and switch means for at will coupling said microphone to said means for reproducing at all said student stations.

a group of student stations each including an electrical musical instrument providing first electrical signals when said instrument is played by a student,

a teacher station including a source of further electrical signals representative of music to be imitated by said students on said electrical musical instruments,

each of said stations having means for reproducing sound represented by electrical signals,

first switch means 41 solely under the control of said teacher for routing the further electrical signals produced at said teacher station to said means for reproducing at all of said student stations, and

second means (61, 62) solely under the control of said teacher for concurrently routing said first electrical signals to said means for reproducing at said teacher's station selectively either concurrently from all said student stations of said group or singly from any one of said student stations of said group, wherein said first means includes means (671 to 6) for selectively routing the electrical signals produced at said teacher station to only any selected one of said student stations and for concurrently discontinuing routing (67 cancel button) of said signals to all said student stations.

30. In a music teaching laboratory,

an audio power amplifier having an input circuit and an output circuit, said output circuit having a low source impedance and providing plural watts of power,

an electrical musical instrument having a source of music signals connected to said input circuit,

a loudspeaker,

switch means connecting said loudspeaker at will to said output circuit,

headphones having a predetermined low impedance,

a relatively high impedance resistive network connectable by said switch means from said output circuit to said headphones, said high impedance network including resistance of at least 40 times that of said headphones,

an outgoing audio line connected through said resistive network to said headphones,

a relatively low impedance connected between said outgoing line and ground,

an incoming line,

a second relatively high resistive network connected between said incoming line and said headphone and having an impedance of the order of magnitude of said first resistive network.

31. In a music laboratory,

a group of student stations,

each of said student stations including an electrical musical instrument providing electrical tone signals representative of music played on said instrument,

an output line,

means for applying said electrical signals to said output line,

an input line,

a sound transducer connected to receive signals from both said output and input lines;

a group amplifier having an input and an output,

switch means (44) for selectively applying concurrently all said signals from all said output lines to said input of said group amplifier and (R7) from said output of said group amplifier to all said input lines, whereby each student hears his own playing from both said input and output lines and the playing of other students only from an input line.

32. The combination according to claim 31 further comprising a teacher station,

a source at said teacher station of further electrical signals representative of music,

further switch means (67-l to 6) at said teacher station for directing said further signals selectively to said input line of any one of said student stations or concurrently (62 A or B or C or D) to said input lines of all said student stations.

said further switch means being arranged to apply said further signals from said teacher station to said input of said group amplifier whereby to concurrently apply said electrical signals to said input lines of all said student stations.

33. in a teaching system for teaching music to a class composed of plural groups of music students, each group including plural students,

a teacher's station having interlocked group selection pushbuttons, one for each group, interlocked individual pupil pushbuttons one for each student of a group, a class selection pushbutton, and a separate mode selection device for each of said groups,

a pupil source of information bearing signal available to each of said pupils,

a pupil headset and a microphone available to each of said students, I

a teacher microphone,

a teacher headset,

a teacher source of information bearing signals,

means responsive to actuation of said class button for interconnecting all said pupil headsets with said teacher microphone and said teacher source,

means responsive to actuation of any one of said group buttons for interconnecting only the headsets of the pupils of a corresponding group with said teacher microphone and said teacher source,

means responsive to actuation of any of said individual pupil pushbuttons for connecting only one individual pupil of a group selected in response to actuation of a group selection button with said teacher microphone and said teacher source, and

7 means responsive to actuation of said mode selection switch for determining selectively that the pupil sources and headsets of a group selected by any one of said group selection buttons shall be interconnected with each other in ensemble.

34. in a music laboratory system,

a teacher station having an incoming and an outgoing teacher line,

a teacher musical instrument providing tone signals to said outgoing line,

a headphone coupled simultaneously to both said teacher lines,

a microphone,

means coupling said microphone only to said outgoing line,

and

means decoupling said microphone from said incoming line,

said last means including said headphone.

35. The claim according to claim 34, wherein is further provided a plurality of pupil stations each including an electronic musical instrument, each of said pupil stations including a pupil incoming line and an outgoing line connected to said instrument, a headphone coupled simultaneously to both the outgoing and incoming pupil lines, a microphone, means coupling said microphone only to said outgoing line and means decoupling said microphone from said incoming line, said last means including said headphone.

36. The combination according to claim 35, wherein is included means for at will concurrently connecting said teacher outgoing line with any one only of said pupil incoming lines and said teacher incoming line only with the outgoing line of said any one only of said pupil incoming lines,

37. The combination according to claim 36, wherein is included means for at will connecting said teacher outgoing line with a preselected group only of said pupil incoming lines and said teacher incoming line with said preselected group only of said pupil outgoing lines, said last means including plural switches operated by a single pushbutton.

38. The combination according to claim 37, wherein is further included means for additionally intercoupling all said pupil incoming lines with all said pupil outgoing lines of said group, said last means including plural switches selected by aan outgoing signal line connected to the instrument at that station,

a separate first pushbutton operatively associated with each said secondary stations and located at said teacher station,

a further pushbutton located at said teacher station and operatively associated with all said student stations,

a rotatable knob,

said first pushbuttons being interlocked to enable actuation of only one at a time,

means responsive to actuation of any selected one of said first pushbuttons for interconnecting the incoming line of said teacher station with a corresponding outgoing line of a selected one of said pupil stations, and the incoming line of the selected one of said pupil stations to the outgoing line of said teacher station,

means responsive to operation of said further pushbutton for connecting the incoming line of said teacher station to all said outgoing lines and the outgoing line of said teacher station to all said incoming lines, and

means operative only in a selected position of said rotatable knob for interconnecting each of said incoming lines to all said outgoing lines.

41. The combination according to claim 40, wherein said last means includes a common amplifier for coupling all said incoming lines to all said outgoing lines.

42. A system of class music instruction from a music teacher station remote from music student stations, said stations each including a musical instrument, including an array of n pushbuttons at said teacher station,

a further array of m pushbuttons at said teacher station,

wherein said students include mn individuals,

m and n being integers each greater than unity,

means responsive to actuation of any of said n pushbuttons for connecting a predetermined group of m of said student stations with each teacher station, to the exclusion of remaining groups each of m student stations, and

means responsive to further actuation of any one of said m pushbuttons for connecting only one selected student station of the group selected by the actuated n pushbutton for connection to said teacher station,

n rotary switches for selecting for each group of student stations one of at least three available operational modes of teaching, wherein said modes for the separate groups may be either the same or different concurrently. 

1. A system of group instruction in music, comprising a plurality of student stations having musical instruments providing electrical signals representing music, said plurality of stations being comprised of n groups of stations, each group having m stations, where m and n are integers, a headphone for each of said stations, a signal input line for each of said stations, a signal output line for each of said stations, a pair of isolating resistances connected in series between the input line and output line of each of said stations, and a passive circuit connecting each of said headphones to the junction of the resistances of a different one of said pairs of isolating resistances, a separate power amplifier associated with each of said instruments for amplifying said electrical signals, each of said power amplifiers having an output, and means connecting each of said outputs directly to one of said signal output lines.
 2. A system of group instruction, comprising a teacher station, a plurality of student stations divided into groups, each student station being provided with an electrical musical instrument, a set of group pushbutton switches each operable to connect a group of student instruments operatively to said teacher station to the exclusion of all other groups of a plurality of groups, a further class pushbutton switch which when actuated overrides all said group pushbutton switches and connects the instruments of all said groups simultaneously operatively to said teacher station, and a further set of pushbutton switches operative only individually to connect to said teacher station only a single individual of any group selected by one of said group pushbutton switches.
 3. A system of class instruction, comprising a teacher station, a student station including a student electrical musical instrument having tone signal sources, an incoming student signal line proceeding to said student station, an outgoing student signal line connected to said electrical musical instrument to convey said tone signals from said student station, a relatively high resistance network joining said lines, a relatively low resistance headphone connected to ground from a point of said resistance network joined by relatively high resistances of said resistive network to both said lines, and a source of signal emanating from a teacher station connected directly to said outgoing signal lIne, wherein said resistive network and said headphones are arranged and adapted to isolate said lines from each other.
 4. The combination according to claim 3, wherein said teacher station includes an electrical musical instrument providing a teacher musical signal source, teacher headphone at said teacher station, switching means at said teacher station for at will connecting said outgoing student signal line to said teacher headphone and said teacher musical signal source to said incoming student signal line.
 5. A system of intercommunication between (1) a group of student stations, of a class, each station provided with a student electrical musical instrument or (2) among individual members of said class or (3) among the entire class and (4) a teacher station provided with an electrical musical instrument, each student station having a separate incoming and outgoing line, comprising a group amplifier, a teacher station incoming line, an n position rotary contact switch means having m pairs of decks, wherein n is a number of stationary switch contacts and m is one more than the number of stations of one group, the groups all having equal numbers of stations, and wherein one of the rotary contacts of said m pairs of decks is connected directly to the input of said group amplifier and another to said teacher station incoming line, and the remainder of said rotary contact pairs connected individually with said outgoing and incoming lines of said student stations, wherein each student station outgoing line is connected to a student electrical musical instrument to convey signals therefrom.
 6. The combination according to claim 5, wherein is provided an isolative resistive mixer network, means connected to corresponding ones of said stationary contacts for at will connecting all the incoming lines of a group of said student stations to said isolative resistive mixer network.
 7. The combination according to claim 6, wherein is provided means connected to corresponding ones of said stationary switch contacts for at will connecting any selected individual incoming line to said teacher station to the exclusion of all other incoming lines.
 8. A system of group instruction in music, comprising a plurality of student stations each including an electrical musical instrument providing student electrical tone signals when played by a student, a teacher station including a further electrical musical instrument providing teacher electrical tone signals when played by a teacher, each of said student stations and said teacher station having a headphone responsive to the electrical tone signals provided by the associated electrical musical instrument, means solely under control of said teacher for routing electrical signals provided by any selected one of said student electrical musical instruments to the headphones of said teacher station and for routing all the electrical signals provided by all said electrical musical instruments to all said headphones, each of said electrical musical instruments including an audio amplifier for transmitting said electrical signals, said amplifiers being each of sufficient power output capability to drive a loudspeaker, a separate loudspeaker individually associated with each of said electrical musical instruments, and means at each of said student stations operative by a student at the station for at will selectively connecting each said loudspeaker or headphones to the output of its individually associated audio amplifier.
 9. In a music teaching laboratory having a plurality of electrical musical instruments providing audio tone signals, an incoming line for each of said instruments, an outgoing line for each of said instruments, a relatively high resistance network connecting each incoming line to each outgoing line and providing isolation therebetween for said audio tone signals, headphones permanently connected betwEen an intermediate point of said resistance network and ground, an amplifier having an input and an output circuit, a loudspeaker, and means for connecting said output circuit selectively directly to said outgoing line or directly to said loudspeaker, said headphones having each relatively low resistance of the order of less than 5 percent of the ohmage of one of said relatively high resistance networks, whereby said headphones provide shunt impedances which enhance the isolation provided by said relatively high resistance networks.
 10. In a music laboratory, an incoming teacher line, a teacher station including a teacher musical instrument providing a source of electrical output signals, an output teacher line connected to said source, relatively high isolating resistance connected between said incoming and output lines, a relatively low input impedance tape recorder for receiving signals from said incoming line, and a connection from said tape recorder to an intermediate point of said relatively high resistances.
 11. The combination according to claim 10, wherein is additionally provided further relatively high isolating resistance connected between said lines in parallel with said first relatively high resistance, and a second reproducing means connected between a midpoint of said further relatively high resistance and ground, said second reproducing means having relatively low resistance to further decouple said lines while providing a preselected signal level to said second reproducing means.
 12. The combination according to claim 10, wherein is provided a plurality of pupil stations each including a signal source and a signal receiver connected respectively to an outgoing student line and an incoming student line, and means for selectively and at will connecting said incoming student lines to said outgoing teacher line and said outgoing student lines to said incoming teacher line.
 13. The combination according to claim 12, wherein said means for selectively and at will connecting is located at said teacher station.
 14. In a music laboratory, an incoming signal line for first signals, an outgoing signal line for second signals, a relatively high resistance network joining said lines and providing isolation therebetween for both said signals, a relatively low resistance headset connected between an approximate midpoint of said resistance network and ground, said relatively low resistance being selected to provide decoupling between said lines when taken in conjunction with said relatively high resistance network, a dummy load connected between said outgoing signal line and ground, said dummy load having an impedance of the order of magnitude of said relatively low resistance, a power amplifier having an input terminal and an output terminal, a loudspeaker, a microphone, a musical instrument pickup, means selectively and at will connecting said microphone or said musical instrument pickup to said input terminal, and means selectively and at will connecting said dummy load or said loudspeaker to said output terminal.
 15. The combination according to claim 14, wherein is provided a plurality of groups of student stations, a plurality of group lines, means connecting each of said group lines to each of said student stations of a group of student stations and means for at will selectively coupling said output of said power amplifier to all said group lines, or to a selected group of group lines or to a selected group line.
 16. The combination according to claim 15, wherein said pupil stations have output lines and said teacher station has headphones, said means for at will connecting output lines of selected student stations to said headphones of said teacher station.
 17. In a music laboratory, a plurality of n student stations, where n is an integer, each of said stations including an electrical musical instrument, an outgoing linE connected to convey electrical signals from said electrical musical instrument, earphones, an incoming line connected to convey electrical signals to said earphones, means for conveying electrical tone signals from an electrical musical instrument at each station to the earphones at said each station at a first level, and means for additionally conveying electrical signals from all of said outgoing lines to the incoming line at each station, whereby each student can hear his own playing distinguishably from the playing of the remaining students.
 18. In a music class training system, a microphone, a source of music signals deriving from an electrical musical instrument, an amplifier having an input circuit and an output circuit, first switch means for selectively connecting said microphone and said source of music signals to said input circuit, second switch means connected to said output circuit, an outgoing signal line, a loudspeaker, means connecting said second switch means selectively to said outgoing signal line and said loudspeaker, an incoming signal line, a pair of relatively high isolating resistances connected between said outgoing and incoming signal lines, and a headset connected between the junction of said resistances and a point of reference potential, said headset being of relatively low resistance.
 19. In a music laboratory, a plurality of student stations each including an electrical musical instrument, a separate outgoing line connected to convey signals from each of said instruments, a headphone at each of said student stations, a separate incoming line at each of said student stations connected to convey signal to said headphone at that station, a passive impedance network means for concurrently conveying signals from an incoming line at each station and from the outgoing line at that station to said headphones while isolating said lines from each other, and means for at will connecting all said outgoing lines concurrently to all said headphones via said incoming lines.
 20. The combination according to claim 19 wherein is provided a teacher''s station, said last means being located at said teacher''s station.
 21. The combination according to claim 20, wherein said teacher station includes a source of music signals, and means connecting said source of music signals selectively at will to any one of said incoming lines or to all said incoming lines.
 22. The combination according to claim 20, wherein said teacher station includes a headphone and means at said teacher station for selectively connecting said headphone to any selected one of said outgoing lines and to all said outgoing lines; whereby said teacher may monitor any one of said students or the ensemble of all said students.
 23. The combination according to claim 21, wherein said plurality of student stations includes plural groups of stations, and wherein said teacher station includes means connecting said source of music signals to any group of said stations to the exclusion of the other groups.
 24. The combination according to claim 22, wherein said plurality of student stations includes plural groups of stations and wherein said teacher station includes means for connecting said source of music signals and said headphone located at said teacher station selectively to any one of said groups of stations.
 25. In a music teaching laboratory, an audio power amplifier having an input and an output, said output having a low source impedance and providing plural watts of power, a loudspeaker, a source of music signals deriving from a musical instrument, switch means connecting said loudspeaker at will to said output, headphones having impedance of the order of 4 ohms, a resistive network connectable by said switch means from said output to said headphones, said resistive network having impedance of about 220 ohms, an ouTgoing audio line, a resistance of the order of the resistance of said loudspeaker connected between said outgoing line and ground, an incoming line, a second resistive network of about 220 ohms connected between said incoming line and said headphones.
 26. In a music teaching laboratory, a plurality of student stations arranged in a group, each of said stations including a source of electrical tone signals deriving from a musical instrument, a separate outgoing line to convey signals from each of said sources, a headphone a separate incoming line to convey signals to each of said stations, a passive network for conveying signals from an incoming line at each station and from the outgoing line at that station to said headphones while isolating said lines from one another; and means for at will connecting all said outgoing lines from said stations of said group to all incoming lines in said group.
 27. A system of group instruction in music, comprising a group of student stations each including an electrical musical instrument providing first electrical signals when said instrument is played by a student, a teacher station including a source of further electrical signals representative of music to be imitated by said students on said electrical musical instruments, each of said stations having means for reproducing sound represented by electrical signals, first switch means (41) solely under the control of said teacher for routing the further electrical signals produced at said teacher station to said means for reproducing at all of said student stations, and second means (61, 62) solely under the control of said teacher for concurrently routing said first electrical signals to said means for reproducing at said teacher''s station selectively either concurrently from all said student stations of said group or singly from any one of said student stations of said group, wherein is provided further means (S1- S7, R1- R7) for selectively at will applying electrical signals from each of said student stations to said means for reproducing of all said student stations.
 28. The combination according to claim 27, wherein the source of further electrical signals includes an electrical musical instrument, the combination further comprising a microphone, and switch means for at will coupling said microphone to said means for reproducing at all said student stations.
 29. A system of group instruction in music, comprising a group of student stations each including an electrical musical instrument providing first electrical signals when said instrument is played by a student, a teacher station including a source of further electrical signals representative of music to be imitated by said students on said electrical musical instruments, each of said stations having means for reproducing sound represented by electrical signals, first switch means (41) solely under the control of said teacher for routing the further electrical signals produced at said teacher station to said means for reproducing at all of said student stations, and second means (61, 62) solely under the control of said teacher for concurrently routing said first electrical signals to said means for reproducing at said teacher''s station selectively either concurrently from all said student stations of said group or singly from any one of said student stations of said group, wherein said first means includes means (67-1 to 6) for selectively routing the electrical signals produced at said teacher station to only any selected one of said student stations and for concurrently discontinuing routing (67 cancel button) of said signals to all said student stations.
 30. In a music teaching laboratory, an audio power amplifier having an input circuit and an output circuit, said output circuit having a low source impedance and providing plural watts of power, an electrical musicaL instrument having a source of music signals connected to said input circuit, a loudspeaker, switch means connecting said loudspeaker at will to said output circuit, headphones having a predetermined low impedance, a relatively high impedance resistive network connectable by said switch means from said output circuit to said headphones, said high impedance network including resistance of at least 40 times that of said headphones, an outgoing audio line connected through said resistive network to said headphones, a relatively low impedance connected between said outgoing line and ground, an incoming line, a second relatively high resistive network connected between said incoming line and said headphone and having an impedance of the order of magnitude of said first resistive network.
 31. In a music laboratory, a group of student stations, each of said student stations including an electrical musical instrument providing electrical tone signals representative of music played on said instrument, an output line, means for applying said electrical signals to said output line, an input line, a sound transducer connected to receive signals from both said output and input lines; a group amplifier having an input and an output, switch means (44) for selectively applying concurrently all said signals from all said output lines to said input of said group amplifier and (R7) from said output of said group amplifier to all said input lines, whereby each student hears his own playing from both said input and output lines and the playing of other students only from an input line.
 32. The combination according to claim 31 further comprising a teacher station, a source at said teacher station of further electrical signals representative of music, further switch means (67- 1 to 6) at said teacher station for directing said further signals selectively to said input line of any one of said student stations or concurrently (62 A or B or C or D) to said input lines of all said student stations, said further switch means being arranged to apply said further signals from said teacher station to said input of said group amplifier whereby to concurrently apply said electrical signals to said input lines of all said student stations.
 33. In a teaching system for teaching music to a class composed of plural groups of music students, each group including plural students, a teacher''s station having interlocked group selection pushbuttons, one for each group, interlocked individual pupil pushbuttons one for each student of a group, a class selection pushbutton, and a separate mode selection device for each of said groups, a pupil source of information bearing signal available to each of said pupils, a pupil headset and a microphone available to each of said students, a teacher microphone, a teacher headset, a teacher source of information bearing signals, means responsive to actuation of said class button for interconnecting all said pupil headsets with said teacher microphone and said teacher source, means responsive to actuation of any one of said group buttons for interconnecting only the headsets of the pupils of a corresponding group with said teacher microphone and said teacher source, means responsive to actuation of any of said individual pupil pushbuttons for connecting only one individual pupil of a group selected in response to actuation of a group selection button with said teacher microphone and said teacher source, and means responsive to actuation of said mode selection switch for determining selectively that the pupil sources and headsets of a group selected by any one of said group selection buttons shall be interconnected with each other in ensemble.
 34. In a music laboratory system, a teacher station having an incoming and an outgoing teacher line, a teacher musical instrument prOviding tone signals to said outgoing line, a headphone coupled simultaneously to both said teacher lines, a microphone, means coupling said microphone only to said outgoing line, and means decoupling said microphone from said incoming line, said last means including said headphone.
 35. The claim according to claim 34, wherein is further provided a plurality of pupil stations each including an electronic musical instrument, each of said pupil stations including a pupil incoming line and an outgoing line connected to said instrument, a headphone coupled simultaneously to both the outgoing and incoming pupil lines, a microphone, means coupling said microphone only to said outgoing line and means decoupling said microphone from said incoming line, said last means including said headphone.
 36. The combination according to claim 35, wherein is included means for at will concurrently connecting said teacher outgoing line with any one only of said pupil incoming lines and said teacher incoming line only with the outgoing line of said any one only of said pupil incoming lines.
 37. The combination according to claim 36, wherein is included means for at will connecting said teacher outgoing line with a preselected group only of said pupil incoming lines and said teacher incoming line with said preselected group only of said pupil outgoing lines, said last means including plural switches operated by a single pushbutton.
 38. The combination according to claim 37, wherein is further included means for additionally intercoupling all said pupil incoming lines with all said pupil outgoing lines of said group, said last means including plural switches selected by a single switch actuator additional to said single pushbutton.
 39. The combination according to claim 38, wherein said last means includes only one amplifier having its input connected commonly to all said outgoing lines, and its output connected commonly to all said incoming lines.
 40. In a music laboratory, a teacher station including a teacher electrical musical instrument and plural student stations, each including a student electrical musical instrument, each of said stations including an incoming signal line and an outgoing signal line connected to the instrument at that station, a separate first pushbutton operatively associated with each said secondary stations and located at said teacher station, a further pushbutton located at said teacher station and operatively associated with all said student stations, a rotatable knob, said first pushbuttons being interlocked to enable actuation of only one at a time, means responsive to actuation of any selected one of said first pushbuttons for interconnecting the incoming line of said teacher station with a corresponding outgoing line of a selected one of said pupil stations, and the incoming line of the selected one of said pupil stations to the outgoing line of said teacher station, means responsive to operation of said further pushbutton for connecting the incoming line of said teacher station to all said outgoing lines and the outgoing line of said teacher station to all said incoming lines, and means operative only in a selected position of said rotatable knob for interconnecting each of said incoming lines to all said outgoing lines.
 41. The combination according to claim 40, wherein said last means includes a common amplifier for coupling all said incoming lines to all said outgoing lines.
 42. A system of class music instruction from a music teacher station remote from music student stations, said stations each including a musical instrument, including an array of n pushbuttons at said teacher station, a further array of m pushbuttons at said teacher station, wherein said students include mn individuals, m and n being integers each greater than unity, means responsive to actuation of any of said n pushbuttons foR connecting a predetermined group of m of said student stations with each teacher station, to the exclusion of remaining groups each of m student stations, and means responsive to further actuation of any one of said m pushbuttons for connecting only one selected student station of the group selected by the actuated n pushbutton for connection to said teacher station, n rotary switches for selecting for each group of student stations one of at least three available operational modes of teaching, wherein said modes for the separate groups may be either the same or different concurrently. 